JP6632266B2 - Imaging device - Google Patents

Description

  The present invention relates to an imaging device that captures an optical coherence tomographic image.

  2. Description of the Related Art An imaging apparatus (hereinafter, referred to as an OCT apparatus) using optical coherence tomography (hereinafter, referred to as an OCT) has been developed. An OCT apparatus irradiates an object with light, changes the wavelength of the irradiating light, causes the reference light and reflected light returning from different depths of the object to interfere with each other, and the intensity of the interference light (hereinafter, abbreviated as interference spectrum). The tomographic image of the object can be obtained by analyzing the frequency components included in. The OCT apparatus is used for, for example, a fundus examination.

  In fundus examination, there is a basic requirement to find a lesion located away from the macula in order to detect an eye disease early and start treatment. In recent years, in order to meet this demand, an OCT apparatus used for fundus examination is expected to have a wide angle of view. In general, since the eyeball is substantially spherical, the optical path length of irradiation light is largely different between the central part and the peripheral part of the fundus. Therefore, the request for widening the angle simultaneously raises a request to collectively obtain a tomographic image of a desired depth range of the fundus at a time.

  This is because, in an OCT apparatus using a wavelength-swept light source (Swept Source OCT apparatus, hereinafter referred to as an SS-OCT apparatus), a time-varying interference signal detected as an analog signal by an optical detector is converted to an A / D converter. This is equivalent to sampling at a higher rate. In order to generate a sample clock of a general SS-OCT apparatus, it is widely used to separately configure a clock generation interferometer having a larger optical path length difference and use the output in addition to an interferometer that generates interference light to be sampled. Have been done. In order to increase the sample rate in such an SS-OCT apparatus, it is necessary to further increase the optical path length difference of the clock generation interferometer. That is, it is necessary to use an expensive wavelength-swept light source capable of generating light having a long coherence length that interferes even if the optical path length difference is long. In addition, to perform such high-speed sampling, an expensive A / D converter having a wide analog band had to be used.

  In order to cope with the above-mentioned wide angle, Patent Literature 1 discloses a general configuration of the SS-OCT apparatus having the above-described general sample rate and a plurality of tomographic images in order to widen the observation region of the tomographic image of the fundus. It discloses that a wide range of tomographic images is constructed by joining.

  On the other hand, Patent Document 2 exemplifies a configuration in which an interleaving operation is performed using two A / D converters in order to configure an OCT device, although this is not a measure for the above-described problem of widening the angle of view.

JP 2012-115578 A JP 2014-016181 A

  However, in Patent Document 1, it is not necessary to use an expensive wavelength-swept light source capable of generating light having a long coherence length and an expensive A / D converter having a wide analog band, but image processing for joining is not required. Has the disadvantage that it takes time and effort. Patent Literature 2 only exemplifies a configuration in which an interleaving operation is performed, and uses the configuration to solve a problem of obtaining a tomographic image in a depth range comprehensively at a time with widening of the angle at a time. It does not disclose a method for correctly maintaining the required wavenumber of the sampling timing during the clock transmission process. Therefore, in the configuration of the conventional OCT apparatus, it is difficult to comprehensively obtain a tomographic image of a desired depth range of the fundus at a time when a wide range of the fundus is scanned.

  In view of the above-described problem, an object of the present invention is to provide a sampling method for comprehensively acquiring a tomographic image of a desired depth range of the fundus when one scan is performed over a wide range of the fundus. That is.

One of the imaging devices according to the present invention is:
The light emitted from the light source section that sweeps the wavelength of the emitted light is split into irradiation light and reference light that irradiate the fundus, and the reflected light from the fundus irradiated with the irradiation light interferes with the reference light. Interference part,
A scanning unit that scans the irradiation light at the fundus,
A detection unit that detects interference light obtained by interference by the interference unit,
A conversion unit that converts an analog signal obtained by detecting the interference light by the detection unit into a digital signal,
An optical path through which a part of the light emitted from the light source unit passes is configured as an interferometer branched into a first optical path and a second optical path having an optical path length difference with respect to the first optical path, and the conversion is performed. A clock generating unit that generates a clock that is a clock for sampling the analog signal at a frequency corresponding to the optical path length difference, and that alternately repeats rising and falling at predetermined wavenumber intervals ;
An imaging apparatus comprising: a tomographic image acquisition unit configured to acquire a tomographic image of the fundus, based on the digital signal obtained by converting the analog signal sampled by the conversion unit using the generated clock,
The scanning unit is configured such that the angle at which the irradiation light is scanned in the fundus becomes 47 degrees or more in air.
The converting unit is a two clock obtained from a clock of a frequency corresponding to the previous Kihikariro length difference, the other with respect to one of the two clocks repeating rise and fall alternately at predetermined wave number intervals Circuit, a first A / D converter for sampling the analog signal using the one, and a second A / D converter for sampling the analog signal using the inverted one. / D converter, wherein the first A / D converter and the second A / D converter sample the analog signal so that the frequency is higher than the frequency corresponding to the optical path length difference. The analog signal is sampled by the clock.

  According to the present invention, it is possible to provide a high-speed and inexpensive sampling method for comprehensively acquiring a tomographic image of a desired depth range of the fundus when one scan is performed over a wide range of the fundus. .

Schematic diagram showing an example of the OCT apparatus according to the present embodiment FIG. 2 is a schematic diagram illustrating an example of a method of scanning irradiation light performed by a scanning unit of the OCT apparatus according to the present embodiment. Schematic diagram of the eyeball according to the present embodiment FIG. 7 is a diagram for explaining a problem of widening the angle of view according to the present embodiment FIG. 6 is a diagram for explaining an optical frequency change and a k clock of the wavelength tunable light source according to the present embodiment. Schematic diagram of the k clock generation unit according to the present embodiment Diagram for explaining the sampling theorem according to the present embodiment Diagram for explaining interleaving according to the present embodiment FIG. 4 is a diagram for explaining that a clock corresponding to a frequency of k clocks according to the present embodiment is generated as a differential signal. FIG. 2 is a schematic diagram illustrating a circuit configuration of an interleave according to the present embodiment and a state of an interference signal and k clocks.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. It should be noted that the following embodiments do not limit the present invention related to the claims, and all combinations of the features described in the present embodiments are not necessarily essential to the solution of the present invention. Absent. For example, the OCT apparatus according to the present embodiment is configured by a Mach-Zehnder interferometer, but the present invention is not limited to this, and may be configured by a Michelson interferometer. In addition, the OCT apparatus according to the present embodiment is configured to change the reference optical path length, but the present invention is not limited to this, and is configured to change the optical path length difference between the reference light and the measurement light. Just do it. For example, the configuration may be such that the reference optical path length is fixed and the measurement optical path length is changed. In the present embodiment, the light source unit 10 is not particularly limited as long as the light source changes the wavelength of light. For example, an external resonator type wavelength sweep light source using a diffraction grating or a prism, or various external resonator type light sources using a Fabry-Perot tunable filter having a variable resonator length can be used. Alternatively, an SSG-DBR that changes the wavelength using a sampled grating, a wavelength-variable VCSEL using a MEMS mechanism (MEMS-VCSEL), or the like can also be used. Further, a fiber laser can be used. The fiber laser may be of a dispersion tuning type or a Fourier domain mode locked type. In addition, as an external resonator type wavelength sweep light source using a diffraction grating, a prism, or the like, a diffraction grating is provided in a resonator to split light, and a polygon mirror or a stripe-shaped reflection mirror is provided on a rotating disk. And a wavelength-swept light source that continuously changes the wavelength of light to be emitted.

(Configuration of SS-OCT device)
FIG. 1 is a diagram illustrating a configuration example of an imaging apparatus (OCT apparatus) using optical coherence tomography according to the present embodiment. The OCT apparatus includes a light source unit 10 from which an emitted optical frequency is swept, an OCT interference unit 20 that generates interference light, a detection unit 30 that detects interference light, and a fundus of the subject 100 based on the interference light. And an information acquisition unit 40 for acquiring the information of the above. The information acquisition unit 40 also functions as a tomographic image acquisition unit (image generation unit) that acquires (generates) a tomographic image of the fundus. Further, the OCT apparatus has a measurement arm 50 and a reference arm 60.

  The OCT interference section 20 has couplers 21 and 22. First, the coupler 21 splits the light emitted from the light source unit 10 into irradiation light for irradiating the fundus and reference light. The irradiation light is applied to the subject 100 via the measurement arm 50. More specifically, the irradiation light that has entered the measurement arm 50 is emitted as spatial light from the collimator 52 after the polarization state is adjusted by the polarization controller 51. Thereafter, the irradiation light is applied to the fundus of the subject 100 via the X-axis scanner 53, the Y-axis scanner 54, and the focus lens 55. Note that the X-axis scanner 53 and the Y-axis scanner 54 are scanning units having a function of scanning the fundus with irradiation light. The irradiation position of the irradiation light on the fundus is changed by the scanning unit. Then, the backscattered light (reflected light) from the fundus is again emitted from the measurement arm 50 via the focus lens 55, the Y-axis scanner 54, the X-axis scanner 53, the collimator 52, and the polarization controller 51. Then, the light enters the coupler 22 via the coupler 21.

  On the other hand, the reference light enters the coupler 22 via the reference arm 60. More specifically, the reference light that has entered the reference arm 60 is adjusted in polarization state by the polarization controller 61 and then emitted from the collimator 62 as spatial light. Thereafter, the reference light passes through the dispersion compensating glass 63, the optical path length adjusting optical system 64, and the dispersion adjusting prism pair 65, enters the optical fiber via the collimator lens 66, exits from the reference arm 60, and enters the coupler 22.

  The reflected light of the subject 100 passing through the measurement arm 50 and the light passing through the reference arm 60 interfere with each other at the coupler 22. Then, the detecting unit 30 detects the interference light. The detection unit 30 includes a differential detector 31 and a conversion unit 32 including n (n is an integer of 2 or more) A / D converters. First, in the detection unit 30, the interference light that has been demultiplexed immediately after the coupler 22 generates the interference light is detected by the differential detector 31. Then, the OCT interference signal converted into an electric signal (analog signal) by the differential detector 31 is converted into a digital signal by the conversion unit 32. Then, the digital signal is sent to the information acquisition unit 40, and the digital signal is subjected to frequency analysis such as Fourier transform, thereby obtaining fundus information. The obtained fundus information is displayed on the display unit 70 as a tomographic image.

  Here, the imaging apparatus according to the present embodiment preferably further includes an analysis unit that performs segmentation on a plurality of layers by analyzing the acquired tomographic image. For example, the information acquisition unit 40 may function as an analysis unit. Is also good. At this time, it is preferable to further include an image generation unit that generates a planar image along any one of the plurality of layers based on the analysis result of the analysis unit. For example, the information acquisition unit 40 functions as an image generation unit. You may let it. Then, a display in which the planar image and the tomographic image are displayed on the display unit 70 in a state where the positional relationship between the macula and the optic disc of the fundus included in the planar image and the macula and the optic disc of the fundus included in the tomographic image is associated with each other. It is preferable to further include a control unit. For example, the information acquisition unit 40 may function as a display control unit. Accordingly, a planar image along any one of the plurality of layers can be observed in a wide angle of view range, so that diagnostic efficiency and diagnostic accuracy can be improved. It is also preferable to further include a calculation unit that calculates the curvature information of the fundus including the macula and the optic disc of the fundus using a tomographic image of the macula and the optic disc of the fundus. For example, the information acquisition unit 40 functions as a calculation unit. You may let it. Thereby, since the curvature of the fundus in a wide angle of view range can be quantitatively evaluated, diagnosis efficiency and diagnosis accuracy can be improved. When acquiring a tomographic image including the macula and the optic disc of the fundus, the scanning unit may be controlled so that the irradiation light is irradiated to the macula and the optic disc in one scan. After obtaining a three-dimensional tomographic image of the fundus, a tomographic image including the macula and the optic disc may be reconstructed from the three-dimensional tomographic image.

  The sampling of the interference light is performed based on a clock at equal optical frequency (equal wave number) intervals, that is, a k-clock signal transmitted from a k-clock generator 80 provided outside the light source unit 10. In Fourier domain OCT including SS-OCT, it is indispensable to perform sampling at the same wave number in order to obtain image quality sufficient for diagnosis. Therefore, in the SS-OCT, it is general to provide the k clock generation unit 80 as in the present embodiment. Even if the k-clock is not used, for example, if the optical frequency emitted from the light source unit 10 changes accurately and linearly with time, sampling at equal time intervals does not cause any problem. However, as schematically shown in FIG. 5A, in a general wavelength-variable light source, the wavelength is swept by changing the resonator length by a driving mechanism, so that not only becomes nonlinear with respect to time. Stable running is very difficult. In addition, since an optical frequency shift due to a mode hop (a phenomenon in which the wavelength changes discontinuously at a certain timing) sometimes occurs, it is difficult to calculate accurate distance information by sampling at equal time intervals. I can say. However, the present invention is not limited to this. For example, after sampling at equal intervals and performing interpolation or the like, the wave number space may be converted to the real space. A coupler 90 is provided to branch a part of the light emitted from the light source unit 10 to the k clock generation unit 80. The k clock generation unit 80 and the coupler 90 are incorporated in the light source unit 10. You may.

  The above is the process of acquiring information on a tomographic slice at a certain point on the subject 100. Acquiring information on a tomographic slice in the depth direction of the subject 100 in this manner is called an A-scan. In addition, the scanning direction for acquiring information on the tomogram of the subject in a direction orthogonal to A-scan, that is, a scanning direction for acquiring a two-dimensional image is orthogonal to any of the scanning directions of B-scan, A-scan, and B-scan. Scanning in the direction is called C-scan. This is because, when performing two-dimensional raster scanning on the fundus oculi when acquiring a three-dimensional tomographic image, the high-speed scanning direction is B-scan, and the low-speed scanning direction in which B-scan is arranged in the orthogonal direction is C-scan. Call it scan. By performing A-scan and B-scan, a two-dimensional tomographic image can be obtained. By performing A-scan, B-scan, and C-scan, a three-dimensional tomographic image can be obtained. The B-scan and the C-scan are performed by the X-axis scanner 53 and the Y-axis scanner 54 described above.

  Note that the X-axis scanner 53 and the Y-axis scanner 54 are each configured by a deflecting mirror arranged so that the rotation axes are orthogonal to each other. The X-axis scanner 53 performs scanning in the X-axis direction, and the Y-axis scanner 54 performs scanning in the Y-axis direction. Each of the X-axis direction and the Y-axis direction is a direction perpendicular to the direction of the eye axis of the eyeball, and is a direction perpendicular to each other. Further, the line scanning direction such as B-scan or C-scan does not need to coincide with the X-axis direction or the Y-axis direction. For this reason, the B-scan and C-scan line scanning directions can be appropriately determined according to the two-dimensional tomographic image or the three-dimensional tomographic image to be captured.

  In addition, various scans can be performed by driving both the X-axis scanner 53 and the Y-axis scanner 54 and changing the angle of the deflecting mirror. For example, raster scanning as shown in FIGS. 2A and 2B may be used, or a method of passing a point of an eyeball (for example, a macula) a plurality of times as shown in FIG. 2C may be used. Further, as shown in FIG. 2D, the scanning may be performed spirally around one point (for example, the macula) of the eyeball.

(Scan angle)
As described above, there is a demand for wide-angle imaging in fundus examination. A range (scanning angle) of scanning with irradiation light of the OCT apparatus required to realize this will be described using a simple human eye model in FIG. FIG. 3 is a schematic diagram when the eyeball is assumed to be a sphere. The macula is located on the visual axis substantially along the center of the pupil of the eyeball at the fundus, and the optic disc is located slightly away from the macula. The macula and optic disc are particularly important sites on the fundus. The wide angle means that the macula, the optic papilla and the periphery thereof are imaged at once by the same scan.

  In a standard adult fundus, the distance D between the macula and the optic disc is about 5.75 mm. The irradiation light is turned around the center of the pupil of the eyeball and scans the fundus. When the range including the optic papilla is imaged by the same scanning centering on the macula, the length (imaging range) L of the shortest curve connecting the macula and the optic papilla is required to be about 14 mm in consideration of individual differences. Here, the deflection angle of the measurement light turned around the pupil center corresponding to the imaging range is defined as α. Since the average of the diameters of the adult eyeballs is about 24 mm, the deflection angle α needs to be 33.4 degrees or more to make the imaging range L 14 mm or more. Using the fact that the average refractive index in the eyeball is 1.38, the angle is expressed as arcsin (1.38 × sin (33) 0.4 degrees / 2)) × 2 ≒ 47 degrees. In other words, in order to simultaneously image the macula and the optic disc around the macula, when the fundus is linearly scanned with irradiation light, the angle range over which the fundus is scanned should be 47 degrees or more in air. I just need. In the following, a range obtained by converting the angle range in which the fundus is scanned in the air when the fundus is linearly scanned with the irradiation light is defined as an angle of view. That is, the deflection angle β is set as the angle of view.

Next, a problem that occurs when scanning at the above-mentioned deflection angle β will be described with reference to FIG. FIG. 4 is a schematic diagram when the eyeball is assumed to be a sphere similarly to FIG. The dashed line in FIG. 4 represents the scanning trajectory. As shown in FIG. 4, the physical distance from the center of the pupil to the outer wall of the eyeball, that is, the fundus is a + b in the macula, and a in a portion distant from the macula (the angle θ / 2 portion). a and b are expressed by the following equations using the length T of the axial length of the eye and the shake angle θ in the eyeball.
a = T × cos (θ / 2) Equation 1
a + b = T Equation 2
Thus, the distance from the center of the pupil to the macula differs from the distance from the center of the pupil to a position away from the macula by b. Since the angle b increases as the angle θ increases, the OCT apparatus for fundus examination with a wide angle of view has an optical path length from the center of the pupil to the macula and a distance from the center of the pupil to a peripheral position away from the macula. Will be significantly different. The eye axis length T of an adult varies greatly between individuals, and the range of the eye axis length T that includes 95% of the persons is 21 mm or more and 28 mm or less. Here, as the value of the axial length T, 28 mm, which is the maximum value in the range, is used. When the shake angle θ in the eyeball is 33.4 degrees, b is approximately 1.2 mm from Equations 1 and 2.

  The fundus tissue observed by the OCT device for fundus examination is the retina near the surface of the fundus and the choroid at the back. Since the retina has a thickness of about 0.50 mm at the thickest portion and a choroid of about 0.30 mm, it is necessary for the OCT apparatus for fundus examination to image to a depth of at least 0.80 mm. That is, there is a difference of 0.8 mm between the surface of the fundus and the choroid.

  Accordingly, in order to image the macula and the optic disc with the same scan and obtain information between the vicinity of the surface of the optic disc and the choroid at the back of the macula, a distance of 2 × (b + 0.80) ≒ 4.0 mm A difference is required. This distance difference corresponds to 4.0 mm × 1.38 ≒ 5.5 mm when considered as an optical path difference in the air. That is, when the angle of view is 47 degrees or more, the depth range of the tomographic information (that is, the optical path length difference) that must be obtained is 5.5 mm in the air. If only a depth range smaller than this range can be achieved, there is a problem that image folding occurs around the fundus, and correct imaging cannot be performed. Generally speaking, in conventional OCT apparatuses, the angle at which the fundus is scanned is about 40 degrees in air, and the depth range (the distance in the depth direction of the tomographic image) is the boundary between the choroid and the sclera. The depth range is set to about 2.6 mm in the eyeball and about 3.6 mm in air in order to capture images up to. Therefore, it can be said that it was difficult to widen the angle of view to 47 degrees or more.

(K clock generator)
Next, k clocks for realizing a desired depth range will be described with reference to FIG. The reference numerals in FIG. 6 correspond to those in FIG. For example, a part of the light emitted from the light source is branched by the coupler 90 having a branching ratio of 95: 5 into the k clock generating unit, and the branched light is incident. The split light is further split into two optical paths by the coupler 81, and the split optical paths are configured as a first optical path and a second optical path. An optical path length difference 82 is provided between the first optical path and the second optical path, and the optical path is caused to interfere again by the coupler 83. The above is the description of the k-clock interferometer. The k-clock interference signal generated here becomes a sine wave with the time change of the optical frequency, and the period of the sine wave also changes with the time of the change of the optical frequency. Since the zero-cross point or peak point of the sine wave shown in FIG. 5B is at an equal wave number interval, an OCT interference signal in a wave number space can be obtained by sampling using these points as clock positions.

  The k clock interference signal is subjected to amplitude correction using an amplifier or the like to an appropriate amplitude and voltage as a sample clock of the conversion unit 32 that samples the OCT interference signal.

  Here, since the frequency of the k clock is the sampling frequency of the OCT interference signal, it is selected based on the sampling theorem. For example, when the frequency of the OCT interference signal is less than half the frequency of the k clock as shown in FIG. 7A, the original signal can be reproduced, but as shown in FIG. If so, a false signal will be obtained. For this reason, the frequency of the k clock must be at least twice the frequency of the OCT interference signal.

  In order to give a desired frequency to the k clock generated from the output of the k clock interferometer using the same light source as the OCT interference signal, an appropriate optical path length difference may be given to the k clock interferometer. The sample optical path of the OCT interferometer is usually constituted by a double path composed of an optical path where the irradiation light irradiates the fundus and an optical path where the reflected light returns from the fundus. On the other hand, as shown in FIG. 6, the optical path of the k-clock interferometer according to the present embodiment is configured by a single path in which light is branched, then the optical path length difference is provided without reflection, and the light is combined again. Have been. Therefore, when it is desired to set the depth range of the tomographic image to be 5.5 mm or more in the air (4.0 mm or more in the eyeball), the optical path length difference 82 of the k clock is four times 5.5 mm, that is, in the air. Needs to be 22 mm or more. That is, it is sufficient to set the range to four times or more.

(Optical path length difference and interleave of interferometer of k clock generation unit)
However, a wavelength-swept light source capable of generating light having such a long coherence length is difficult and very expensive to manufacture, and is not suitable for general ophthalmic equipment. Therefore, the SS-OCT apparatus according to the present embodiment performs sampling by an interleave operation. The interleaving is a technique of sampling one signal using a conversion unit 32 including n (n is an integer of 2 or more) A / D converters. Generally, in the A / D conversion, as shown in FIG. 8A, a signal is obtained only at one of a rising edge and a falling edge of a clock. In the interleaving, as shown in FIG. 8B, for example, two clocks whose phases are shifted by 180 degrees using a phase inversion circuit or the like are supplied to two A / D converters, and both the rising edge and the falling edge are provided. The sampling speed is doubled by acquiring the signal at the edge of. That is, as shown in FIG. 10, the conversion unit 32 divides the clock corresponding to the frequency of the k clock interferometer into n clocks corresponding to n A / D converters, and the divided n clocks Is used to generate a clock having a frequency n times the frequency of the k clock interferometer. FIG. 10 is a schematic diagram illustrating a circuit configuration of an interleave according to the present embodiment and states of an interference signal and k clocks. Therefore, in the present embodiment, if the optical path length difference 82 of the k clock is 11 mm or more, which is twice the depth range of the tomographic image in the air, the depth range of the tomographic image is 5.5 mm or more in the air. (4.0 mm or more in the eyeball).

  Here, as shown in FIG. 10, the interleave control unit for generating the clock of n times frequency provided in the conversion unit 32 may be configured to switch the interleave operation. For example, a plurality of patterns may be configured such that the number of A / D converters to be used is one, two, or four, and switching control of each pattern may be performed according to the depth range of the tomographic image. .

  As described above, in the SS-OCT apparatus, it is very important to obtain interference signals at equal wave number intervals in order to improve image quality. However, even when k clocks are generated at equal wave number intervals using a k clock interferometer, a higher clock frequency is required as the depth range is expanded, and the transmission process is increased. In this case, a problem that the uniformity is hindered occurs. The clock frequency can be halved by performing the above-described interleaving operation.However, when transmitted as a single-ended signal, if the signal waveform is distorted during transmission and a phase shift occurs, the homogeneity is lost, and the This is a factor that causes noise (FIG. 9A).

  Therefore, in order to reduce the influence of waveform distortion during transmission, the k clock is transmitted as a differential signal 85 such as an LVDS (Low Voltage Differential Signaling) signal, that is, one signal is transmitted through two signal lines. Is desirable (FIGS. 9B and 9C). By connecting the two signal lines as a pair of signal lines with a phase difference of 180 degrees on the + side and the-side, the potential difference between the signal lines becomes a signal level, so that external common-mode noise is canceled out. 9 (d)). Further, by using the differential signal 85, the slew rate is doubled, and the timing uncertainty is halved compared to the single-ended signal. Therefore, transmission can be performed with higher accuracy while maintaining equal wavenumber. FIG. 9 is a diagram for explaining that a clock corresponding to the frequency of the k clock according to the present embodiment is generated as a differential signal.

Here, the necessary sample frequency is shown using actual numerical values that can be provided in the present embodiment. Assuming that the depth range of the tomographic image (measurement distance) Δz, the center wavelength λc, and the sweep wavelength width Δλ, the number of samples N, which is the number of sampling the entire depth range of the tomographic image at a time, is (4 × Δz × Δλ). ) / Λc ² . On the other hand, assuming that the frequency fA of the wavelength sweep is a duty ratio (a period during which light emission is effective as OCT during one sweep) d, the frequency fs of the k clock is derived from (N × fA) / d. In the light source according to the present embodiment, for example, λc = 1040 nm, Δλ = 110 nm, fA = 100 kHz, and d = 0.446.

When the depth range of the tomographic image is 5.5 mm in the air (4.0 mm in the eyeball), the sample number N is derived as (4 × 5.5 × 10 ⁶ × 110) / 10402 ² = 2237. Is done. In this case, the frequency fs of the k clock is derived as (2237 × 100 × 10 ³ ) /0.446=501.57 MHz. Here, since sampling is performed by interleaving using two A / D converters, the clock frequency for one A / D converter is 501.57 MHz / 2 = 250.64 MHz.

When the present embodiment is applied to the configuration of the conventional commercially available OCT apparatus described above, the following may be considered. When the depth range is 3.6mm in conventional air (2.6 mm in the eye), the number of samples is derived as ^{(4 × 3.6 × 10 6 ×} 110) / 1040 2 = 1464. In this case, the frequency fs of the k clock is derived as (1464 × 100 × 10 ³ ) /0.446=328.25 MHz. According to the present embodiment, the interleaving using two A / D converters is performed. , The number of samples is 1464 × 2 = 2928, and the frequency fs of the k clock is 328.25 × 2 = 656.5 MHz. Therefore, a tomographic image of 7.2 mm in the air (5.2 mm in the eyeball) can be obtained.

  As described above, according to the configuration of the present embodiment, a tomographic image required for wide-angle imaging at an angle of view of 47 degrees or more without deterioration in image quality causing a phase shift due to signal waveform distortion during clock signal transmission. The clock frequency per A / D converter is 5.5 mm in the air with a depth range of 5.5 mm (4.0 mm in the eyeball), and the clock frequency is 250.64 MHz. At a frequency lower than 328.25 MHz. Even if the clock frequency for one A / D converter is 328.25 MHz, which is the frequency used in a conventional OCT apparatus, 7.2 mm in the air (5.2 mm in the eyeball) is used. This means that an image can be obtained. That is, when one scan is performed over a wide range of the fundus, a tomographic image of a desired depth range of the fundus can be comprehensively acquired.

  The present invention is not limited to the case where one scan is performed over a wide range of the fundus. That is, the present invention is not limited to the case where the scanning unit is configured to scan the irradiation light on the fundus at an angle of 47 degrees or more in air. Regardless of the scanning angle, when it is desired to obtain a tomographic image of the fundus within a range of 4.0 mm or more in the depth range of the eyeball, the k clock generation unit considers interleaving sampling, and What is necessary is just to be comprised so that it may become an optical path length difference corresponding to the range which becomes 4.0 mm or more. At this time, as described above, the k clock generating unit performs interleaving using two clocks whose phases are shifted by 180 degrees and two A / D converters, in which the second optical path is configured by a single path. If sampling is performed, it is preferable that the optical path length difference be 11 mm or more in air. In addition, it is preferable that the k clock generation unit be configured such that the optical path length difference becomes 5.5 mm or more in air if the second optical path is configured by a double path. Further, the same effect can be obtained even if the k clock generation unit is configured to artificially change the number of samplings and the frequency of the clock instead of setting the optical path length difference to these lengths.

  The optical path length difference 82 of the k clock may be 13.8 mm in the air in the case of a single pass, and 6.9 mm in the air in the case of a double pass. At this time, the depth range of the tomographic image can be configured to be 6.9 mm or more in air (5.0 mm or more in the eyeball). Further, the optical path length difference 82 of the k clock may be 16 mm in the air in the case of a single pass, and may be 8 mm in the air in the case of a double pass. At this time, the depth range of the tomographic image can be configured to be 8.0 mm or more in air (5.8 mm or more in the eyeball).

(Other embodiments)
Further, it is preferable that the imaging device according to the present embodiment further includes a selection unit that selects a plurality of imaging modes having different scanning angles. At this time, the control unit controls the scanning unit to change the scanning angle according to the selected imaging mode. For example, in an imaging mode (wide-angle imaging mode) for imaging as a tomographic image including both the macula and the optic disc, the scanning angle is set to a first angle of 47 degrees or more, and one of the macula and the optic disc is set. In the imaging mode for capturing images (narrow angle of view imaging mode), the angle can be changed to a second angle of less than 47 degrees.

  In addition, at this time, the selection unit changes the depth range from the first range of 4.0 mm or more to the second range of less than 4.0 mm in the eyeball at the same time based on the change of the scanning angle. The configuration of the k clock generator for changing the depth range will be described below with reference to FIG.

  As described above, the frequency of the k clock corresponds to the optical path length difference 82 of the k clock interferometer. Therefore, in the second optical path having an optical path length difference with respect to the first optical path, for example, a material (a gas or the like) whose refractive index can be changed is provided, and light is once emitted from the fiber into the air and returned to the fiber. In the above-described configuration in which the optical distance between the fiber ends is changed, or the light is once emitted out of the fiber, the folding mirror provided on the movable stage is moved in the optical axis direction by using a plurality of folding mirrors. For example, the optical path length difference can be changed by a method such as the following.

  At this time, the control unit controls the scanning unit and the changing unit such that the scanning angle and the optical path length difference are changed according to the selected imaging mode. It is preferable that the change unit is configured to be able to change from a first optical path length difference of 11 mm or more to a second optical path length difference of less than 11 mm, for example, when an interleave operation is used. The success of the changing unit is also used for switching the depth range of the tomographic image accompanying the switching of the interleaving operation in the interleave control unit in the above-described tomographic image acquiring unit.

  Furthermore, the selection unit may be configured to be able to select any one of a plurality of imaging modes including an imaging mode (vitreous observation mode) for imaging so as to include the vitreous, retina, and choroid of the eye. good. At this time, when an imaging mode for imaging so as to include the vitreous body, retina, and choroid of the eye is selected, for example, a tomographic image of the fundus in a range of 4.0 mm or more in the eyeball in the depth range is acquired. Is preferred. At this time, it is not essential to widen the imaging range, but it is effective for switching the depth range in the main imaging mode. This is because, in order to perform tomographic imaging so that the vitreous body, retina, and choroid of the eye are sufficiently included, a depth range of 4.0 mm or more in the eyeball is required.

  When the depth range to be imaged is changed by selecting the mode in any of the imaging modes, the k-clock generation unit simulates the number of samplings and the frequency of the clock described later instead of changing the optical path length difference. The same effect can be obtained even if it is configured to change to. Here, the term “simulated” means that, for example, when the depth range is narrowed, the k clock generation unit may be configured to reduce the number of samplings and the frequency of the clock. There is an effect that the calculation time of the distance data of the A-scan can be shortened by simply reducing the number.

  The present invention is also realized by executing the following processing. That is, software (program) that realizes the functions of the above-described embodiments is supplied to a system or apparatus via a network or various storage media, and a computer (or CPU, MPU, or the like) of the system or apparatus reads the program and reads the program. This is the process to be performed.

Claims (15)

The light emitted from the light source section that sweeps the wavelength of the emitted light is split into irradiation light and reference light that irradiate the fundus, and the reflected light from the fundus irradiated with the irradiation light interferes with the reference light. Interference part,
A scanning unit that scans the irradiation light at the fundus,
A detection unit that detects interference light obtained by interfering with the interference unit,
A conversion unit that converts an analog signal obtained by detecting the interference light by the detection unit into a digital signal,
An optical path through which a part of the light emitted from the light source unit passes is configured as an interferometer branched into a first optical path and a second optical path having an optical path length difference with respect to the first optical path, and the conversion is performed. A clock generating unit that generates a clock that is a clock for sampling the analog signal at a frequency corresponding to the optical path length difference, and that alternately repeats rising and falling at predetermined wavenumber intervals ;
An imaging apparatus comprising: a tomographic image acquisition unit configured to acquire a tomographic image of the fundus, based on the digital signal obtained by converting the analog signal sampled by the conversion unit using the generated clock,
The scanning unit is configured such that the angle at which the irradiation light is scanned in the fundus becomes 47 degrees or more in air.
The converting unit is a two clock obtained from a clock of a frequency corresponding to the previous Kihikariro length difference, the other with respect to one of the two clocks repeating rise and fall alternately at predetermined wave number intervals Circuit, a first A / D converter for sampling the analog signal using the one, and a second A / D converter for sampling the analog signal using the inverted one. / D converter, wherein the first A / D converter and the second A / D converter sample the analog signal so that the frequency is higher than the frequency corresponding to the optical path length difference. An imaging device configured to sample the analog signal in response to the clock signal.   2. The imaging apparatus according to claim 1, wherein the conversion unit switches a clock for sampling the analog signal by switching the number of A / D converters to be used according to a depth range of the tomographic image. apparatus. The conversion unit samples the analog signal such that a depth range of the tomographic image is switched between a first range of 4.0 mm or more and a second range of the depth range of less than 4.0 mm. The imaging device according to claim 1, wherein a clock having a frequency for switching is switched. The tomographic image acquisition unit, by thinning out the data obtained by the conversion unit is sampled by a clock having a frequency higher than the frequency corresponding to the optical path length difference, Fourier transform the thinned data, the tomographic image The imaging device according to claim 1, wherein a tomographic image having a narrow depth range is obtained. The conversion unit is configured to sample and output the analog signal by the first A / D converter and the second A / D converter, so that a rising edge and a falling edge of a clock having a frequency corresponding to the optical path length difference are generated. The imaging apparatus according to any one of claims 1 to 4, wherein the imaging apparatus is configured to sample the analog signal using both of them . The clock generation unit is one of two clocks obtained from clocks having a frequency corresponding to the optical path length difference, and one of two clocks that alternately rises and falls alternately at a predetermined wavenumber interval. by reversing imaging apparatus according to any one of claims 1 to 5, characterized in that it is configured to generate a differential signal of a clock frequency corresponding to the optical path length difference. Further comprising two signal lines for transmitting two clocks obtained by inverting the other to the one toward the converter;
The two signal lines are connected as one signal line so that noise generated in the two clocks transmitted through the two signal lines is canceled, so that the clock is generated when transmitting the signal lines. The imaging apparatus according to claim 6, wherein a clock that alternately rises and falls at predetermined wave number intervals is transmitted to the conversion unit in a state where noise is reduced. The apparatus according to claim 1, further comprising a control unit configured to control the scanning unit such that the scanning angle is changed between a first angle of 47 degrees or more and a second angle of less than 47 degrees. 8. The imaging device according to any one of items 7 to 7 . Further comprising a selection unit for selecting any one of a plurality of imaging modes including a narrow angle of view imaging mode to image any one of the macula of the fundus and the optic disc,
The imaging device according to claim 8 , wherein the control unit controls the scanning unit so that the irradiation light is scanned at the second angle when the narrow angle-of-view imaging mode is selected. apparatus. The plurality of imaging modes further include a wide-angle imaging mode for imaging the macula of the fundus and the optic disc,
The imaging apparatus according to claim 9 , wherein when the wide-angle imaging mode is selected, the control unit controls the scanning unit so that the irradiation light is scanned at the first angle. apparatus. The conversion unit selects the narrow-field-of-view imaging mode such that when the wide-field-of-view imaging mode is selected, the depth range of the tomographic image is a first range of 4.0 mm or more. and in the case of claim 1 0, characterized in that depth range of the tomographic image so that the second range is less than 4.0 mm, switches the clock for sampling said analog signal Imaging device. The plurality of imaging modes may include a vitreous observation mode for observing the nitric transducer body,
The control unit, when the vitreous observation mode is selected, controls the scanning unit so that the irradiation light is scanned at the second angle,
The converting unit, as the depth range of the tomographic image is in the range is 4.0mm or more, any one of claims 9 to 1 1, characterized in that to switch the clock for sampling the analog signal 2. The imaging device according to claim 1. The converting unit, the image pickup apparatus according to any one of claims 1 to 1 2, characterized in that at equal wave number intervals are configured to sample the analog signal. The tomogram capturing apparatus according to any one of claims 1 to 1 3, characterized in that a 5.0mm or more depth range in the eye. The tomogram capturing apparatus according to any one of claims 1 to 1 4, characterized in that a 5.8mm or more depth range in the eye.

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